Femtosecond Laser-inscribed Multimode Fiber Bragg Gratings

Abstract

Multimode fiber Bragg gratings (MMFBGs) have attracted intensive interests for both sensing and telecom applications due to their unique advantages such as sensitive to bending, multiple wavelengths, and easily coupling with light sources. Both the fabrication technique and coupling mechanism of FBG in multimode fiber have been studied. MMFBGs have also been reported for various kinds of sensing applications such as strain, bending, refractive index, and so on. In this work, we propose and demonstrate the fabrication of MMFBGs using a unique femtosecond laser line-by-line scanning technique. We have fabricated four different types of MFBGs on four different multimode fibers (MMFs), i.e., 105/125μm step-index multimode fiber, 62.5/125μm graded-index multimode fiber, coreless silica fiber, and single-crystal sapphire fiber. The spectral characteristics of MMFBGs with different track lengths, quantities, pitches, and positions were studied. MMFBGs with increased reflectivity were obtained via optimization in fabrication parameters. All of these MMFBGs have exhibit a broad -3dB bandwidth since their spectra consist of multiple reflection peaks. Moreover, the signal-to-noise ratio (SNR) and the peak wavelength will be effected by the cross sectional location of MMFBG. In addition, the high-temperature response of the multimode SFBGs was tested and the experimental results showed the SFBGs could withstand a high temperature of 1612°C. As such, the proposed femtosecond laser-inscribed MMFBGs could be used for high temperature measurements.